1. Field of the Invention
This invention relates to processes for the purification of flue gases by wet methods.
2. History of the Related Art
According to these processes, a washing liquid is brought into contact with the flue gases to be purified in a scrubber. The dust in the gases is transferred to the washing liquid and, at the same time, the gaseous pollutants are absorbed by the washing liquid according to their solubility and the characteristics of this washing liquid. As a large quantity of washing liquid is generally required for the purification of flue gases, the washing liquid is recycled in the scrubber and the pollutants collected are extracted from the scrubber/recycling circuit assembly by means of the smallest possible liquid discharge.
E.g. in the case of refuse incinerating plants, refuse is destroyed and replaced by clinker. The flue gases produced carry the recovered energy and pollutants in particulate and gaseous form, the most common being HCl, HF, SO.sub.2 and the heavy metals in particulate or gaseous form. Once collected, these pollutants must follow one of two routes--reutilization or disposal. A large number of processes have been proposed to condition the residue in order to facilitate disposal or reutilization as building material. The disadvantage of these processes is the necessity to treat large quantities, mixing a small proportion of toxic elements and a large proportion of inert elements.
In this respect, it is advantageous to separate the toxic elements such as the heavy metals from the inert elements prior to specific treatment.
Various processes have been proposed to this end. These usually comprise separation of the dust (or flue dust) in an electrostatic precipitator followed by gas absorption in a scrubber. The liquid discharge of this scrubber is acidic and is used in a separate installation for leaching the flue dust collected in the electrostatic precipitator. It is possible in this manner to produce solid residue leached in the acid phase and a liquid effluent containing heavy metals which are then precipitated into a conventional water treatment plant or extracted selectively for recovery.
The aim of this invention is to eliminate the addition of another complex installation and to make efficient use of the capacity of the scrubber, which, as indicated hereinabove, is of necessity provided with a large recycling circuit for the washing liquid in order to guarantee purification of flue gases. Surprisingly, moreover, experience has shown that leaching effected in a high-capacity recycling circuit of this kind is much easier to control with a view to the acid leaching of the flue dust.